Alter SkCanvas::drawPicture (devirtualize, take const SkPicture, take pointer)

R=reed@google.com, bsalomon@google.com, mtklein@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/313613004
diff --git a/src/gpu/SkGpuDevice.cpp b/src/gpu/SkGpuDevice.cpp
index b05cd33..7514359 100644
--- a/src/gpu/SkGpuDevice.cpp
+++ b/src/gpu/SkGpuDevice.cpp
@@ -1816,7 +1816,7 @@
     return SkSurface::NewRenderTarget(fContext, info, fRenderTarget->numSamples());
 }
 
-void SkGpuDevice::EXPERIMENTAL_optimize(SkPicture* picture) {
+void SkGpuDevice::EXPERIMENTAL_optimize(const SkPicture* picture) {
     SkPicture::AccelData::Key key = GPUAccelData::ComputeAccelDataKey();
 
     const SkPicture::AccelData* existing = picture->EXPERIMENTAL_getAccelData(key);
@@ -1837,11 +1837,11 @@
     result->setPixelRef(SkNEW_ARGS(SkGrPixelRef, (info, texture)))->unref();
 }
 
-void SkGpuDevice::EXPERIMENTAL_purge(SkPicture* picture) {
+void SkGpuDevice::EXPERIMENTAL_purge(const SkPicture* picture) {
 
 }
 
-bool SkGpuDevice::EXPERIMENTAL_drawPicture(SkCanvas* canvas, SkPicture* picture) {
+bool SkGpuDevice::EXPERIMENTAL_drawPicture(SkCanvas* canvas, const SkPicture* picture) {
 
     SkPicture::AccelData::Key key = GPUAccelData::ComputeAccelDataKey();